The present invention relates in general to optical crossconnect (OXC) switches and, in particular, to OXC switch configurations that improve switch design and operation as well as enhancing optical performance.
Mode networks for communicating voice, video and/or data (xe2x80x9ccommunication networksxe2x80x9d) typically include an optical portion and an electrical portion. In the optical portion, information is modulated in an optical signal that is transmitted through optical fibers. In the electrical portion of the network, information is transmitted in the form of an electrical signal via wires. Optical transmission has a number of advantages over electrical transmission, notably, enhanced bandwidth capacity. However, electrical transmission has certain advantages including the widespread availability of electrical network structure and well developed data routing components and protocols. For these reasons, optical fibers are coming to predominate at the network core while wire circuitry remains the standard at the network periphery.
Even within the optical portion of the network, switching is often performed by optical- electrical-optical (OEO) switches. In such switches, the incoming optical signal is converted into an electrical signal, switching is performed in the electrical domain, and the outgoing signal is converted back into the optical domain. OEO switches allow for use of well developed electrical switch technology within the optical portion of the network. However, OEO switches are increasingly becoming the bandwidth bottlenecks of modem communication networks. In addition, such switches generally entail reading routing information from packet headers and the like, and are therefore protocol dependent.
Significant effort has therefore been directed to developing OXC switches for various core and peripheral network applications. OXC switches perform at least some switching functionality by directing optical signals in the form of beams between input and output ports, e.g., fibers or other optical or electro-optical components, without converting the signals into another domain. It will be appreciated that such connections typically support bidirectional communication and the terms xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d are therefore used herein for convenience and not by way of limitation. Such switches can therefore be substantially transparent to the transmitted signals, thereby enhancing bandwidth capabilities and avoiding compatibility issues in connection with new or varying network communication protocols.
Various types of OXC switches have been proposed including fiber translation switches, fiber bending switches and the mirror based switches. In fiber translation switches, one or both of the input and output fibers or fiber bundles are moved or translated in one or more dimensions to align a selected input fiber with a selected output fiber. However, such switches require movement of bulk components and generally are too slow for practical applications in modem communications networks. Moreover, such switches may not allow for multiple, simultaneous and independent connections as between fibers of the moved bundles.
In fiber bending switches, the end of an input and/or an output fiber is bent, e.g., using piezoelectric elements, to optically connect the fibers for transmission of optical beams therebetween. Again, such switches have not gained widespread acceptance for modem communication network applications requiring large-scale switches, fast response times and low insertion losses.
Mirror based switches utilize movable mirrors to redirect optical signals so as to connect a selected input fiber to a selected output fiber. Early designs used a bulk mirror or mirrors with bulk mechanical elements for moving the mirror(s). In cases where one movable mirror interfaces multiple input fibers with multiple output fibers, such switches may not support multiple simultaneous and independent connections. In any event, bulk mirrors generally involve response times that are impractical for modem communication network applications and/or are too large to be inserted into existing racks or other network structures or otherwise have too large a physical footprint to appeal to network providers.
More recently, numerous developers have proposed micro-mirror switches. Typically, these switches are proposed to be implemented using Micro-Electro-Mechanical System (MEMS) technology wherein the mirrors, actuators for moving the mirrors and associated integrated circuitry are fabricated on substrates using semi-conductor fabrication techniques. Micro-mirror switches are promising because it is believed that they will provide acceptable response times, because the mirrors can be mapped to individual fibers to allow for substantially unlimited simultaneous and independent connections (subject to the switch size, e.g., 256xc3x97256), and because practical switches can be dimensioned to appeal to network providers.
However, significant challenges remain with respect to realizing the potential benefits of micro-mirror OXC switches. First, some MEMS designs provide a substantially limited range of angular motion of micro-mirrors, e.g., before the mirrors xe2x80x9cbottom outxe2x80x9d on the substrate. Thus, in order to make large-scale switches, for example, 256xc3x97256 or greater switches, long switch interface path lengths may be required. Long path lengths, in turn, may require a large switch footprint and/or complicated optical folding, may complicate alignment and may entail a risk of cross talk due to beam spreading. Also, accurately controlling the actuators to form any of the possible connections in a large-scale switch is problematic, especially because each mirror typically has a unique switching geometry in conventional designs.
Another practical difficulty is that substantial expense is involved in designing and fabricating MEMS mirror arrays. Because the area or pitch of the mirror array is generally matched to the pitch of the associated fiber array in conventional switch designs, retooling or redesign of the fabrication process may be necessary for each switch geometry. The same limitation may affect the switch footprint.
The present invention relates to improved OXC switch configurations and implementations that address a number of needs as discussed above. In this regard, the invention provides paired movable mirror switch configurations where mirrors in a first array have a common reference orientation relative to a second array and require substantially the same range of angular movement to target the mirrors of the second array thereby simplifying switching control. Additionally, the invention provides configurations for de-coupling the mirror array pitch from the associated fiber array pitch, thereby providing significant flexibility in switch design and implementation. The invention also provides a number of switch geometries for improved optical efficiency.
In accordance with one aspect of the present invention, a number of mirrors of an optical crossconnect switch have a common relative reference orientation with respect to an array of targets such as additional mirrors or ports. In one embodiment, the switch includes two arrays of movable mirrors where any of multiple input ports can be connected to any of multiple output ports via reflection by a mirror of the first array and a mirror of the second array (addition mirrors may be involved in establishing a connection as will be discussed below). In accordance with the present invention, each of a number of mirrors of the first array is configured so as to direct an incident beam to substantially the same location relative to the second array. For example, each of the mirrors of the first array, under a nominal or reference condition, may direct an incident beam from an associated input port to a center point or center mirror of the second array. Thus, the geometry of the first mirrors, or the electrostatic force of the associated actuator components, may be varied from mirror-to-mirror such that the same common mode voltage sets the initial relative tilt angle to properly direct the beam relative to the second array.
It will be appreciated that such an initial relative tilt angle will depend on various geometric factors including the position of the subject mirror in the first array and the incoming beam angle which may be different for different mirrors (i.e., the incoming beams directed from the input ports to the mirrors of the first array may not be parallel). By configuring the mirrors of the first array to have the same relative reference orientation, a larger number of crossconnects (for a given mirror spacing) can be achieved for a given range of active mirror motion. Also, each of the first mirrors of the first array may require substantially the same range of angular motion to address all of the second mirrors of the second array. Moreover, the switch may thereby be readily implemented such that the same control signal applied to any first mirror will establish a connection via the same second mirror, thereby simplifying control.
According to another aspect of the present invention, the pitch of an input or output port array is decoupled from the pitch of an array of movable mirrors. The associated switch includes an array of ports and an array of movable mirrors for directing optical beams relative to (to or from) the ports. The array of ports defines a cross-section port array area or xe2x80x9cport pitchxe2x80x9d with respect to a first plane adjacent to the port array and orthogonal to a first center axis of first beams transmitted relative to the port array. That is, the port pitch is the area of the ports as projected, relative to the first center axis, onto the first orthogonal plane. The array of mirrors defines a cross-sectional mirror array area or xe2x80x9cmirror pitchxe2x80x9d with respect to a second plane adjacent to the mirror array and orthogonal to a second center axis of beams transmitted relative to the mirror array, i.e., the mirror pitch is the area of the mirrors as projected relative to the second center axis onto the second orthogonal plane. It will be appreciated that the ports and/or the mirrors may have a three-dimensional rather than a co-planar configuration. In addition, some or all of the first beams and second beams may have orientations that are not parallel to the respective first and second axes.
In accordance with the present invention, the port pitch may differ from the mirror pitch. For example, the angular orientations of the ports may be varied or a pitch magnifying or demagnifying element (e.g. a mirror or mirrors) may be interposed between the port array and the mirror array to decouple the port pitch from the mirror pitch. Such a pitch translation mechanism allows incoming beams transmitted from the mirrors to the ports to be accepted by port fibers even though the mirrors may not be axially aligned with the fibers. In this manner, a given mirror array design can accommodate various port geometries. Moreover, the switch footprint may be reduced despite constraints as to the dimensions of the port array and/or mirror array. In this regard, it is anticipated that MEMS technology may allow for mirror pitches that are more compact than the port pitches of particular switch designs.
In accordance with a further aspect of the present invention, a fixed mirror is interposed between a mirror array and a port array. The associated switch includes: an array of first ports; an array of second ports; an array of first mirrors where each of the first mirrors is associated with a respective one of the first ports; and an array of second mirrors where each of the second mirrors is associated with a respective one of the second ports. In accordance with the present invention, the switch further includes at least one fixed mirror located either 1) between the first mirrors and first ports or 2) between the second mirrors and second ports. One or more fixed mirrors may be provided at both of the noted positions and the mirror(s) are preferably sufficient for optically interfacing multiple ones of, and up to all of, the first or second mirrors with the first or second ports, respectively. Such fixed mirrors can be used to decouple the port pitch from the associated mirror pitch and/or to amplify the effect of mirror movement and thereby reduce the required range of mirror motion for given mirror/port array dimensions.
According to a still further aspect of the present invention, the angular orientation of a mirror array support structure, such as a substrate in the case of a MEMS mirror array, is established so as to minimize the required tilt angle to effect all possible connections. The term xe2x80x9csubstratexe2x80x9d as used herein means those types of structures that can be handled by the types of equipment and processes that are used to fabricate micro-devices on, within, and/or from the substrate using one or more micro photolithographic patterns. The angle through which an incident signal is reflected by a mirror of an array is dependent on, inter alia, the tilt angle of the mirror relative to the support structure and the angular orientation of the support structure relative to the incoming path of the incident beam. It is desirable to reduce the tilt angle required for the most extreme connections. For example, in the case of connections formed by first and second arrays of movable mirrors, the most extreme connections may be those involving a mirror at one edge of the first array and the opposite edge of the second array and vice versa. Such extreme connections may determine the size of switch that can be achieved for a given mirror spacing, switch geometry and active mirror tilt angle.
In order to reduce the required tilt angle for such connections and potentially enhance the maximum switch size, a switch according to the present aspect of the invention has at least one mirror array formed on a substrate having a particular orientation. Specifically, the switch includes 1) an array of mirrors, tiltable relative to a support structure, for receiving incoming optical beams having a first axis, the mirror array having a first centerpoint, and 2) an array of targets having a second centerpoint, where each mirror of the mirror array can direct an incoming beam to any of the targets and the first and second centerpoints define a second axis. Depending on the switch design, the targets may be ports, mirrors or other optical components. The support structure is oriented such that a third axis, extending through the first centerpoint normal to an upper surface of the support structure, falls between the first and second axes and, preferably, substantially bisects the angle subtended by the first and second axes.
According to another aspect of the present invention, improved optical switch geometries are provided that incorporate one of reflectional symmetry and rotational symmetry. Reflectional symmetry refers to switch configurations where there is at least one axis of symmetry such that an optical pathway across a switch interface is symmetrical about the symmetry axis when projected onto a plane including the symmetry axis. Reflectional symmetry refers to switch configurations where there is at least one axis of symmetry such that an optical pathway across a switch interface is the same, as projected onto a plane orthogonal to the symmetry axis, when rotated 180xc2x0 about the symmetry axis.
In one implementation, the first array is disposed on a first array support structure having a first centerpoint and the second array of mirrors is disposed on a second support structure having a second centerpoint. The first and second structures are oriented such that: 1) a first angle is defined between the line connecting the first and second centerpoints and a line normal to an upper surface of the first structure extending through the first centerpoint; 2) a second angle is defined between the line connecting the first and second centerpoints and the line normal to an upper surface of the second support structure extending through the second centerpoint; and 3) the first angle is substantially the same as the second angle. As will be understood from the description below, such geometry supports both rotational and reflectional symmetry implementations. Such geometries have advantages relating to optical efficiency. In particular, when a beam having a circular cross-section is reflected through an angle, the reflecting beam will have an elliptical cross-section. If the reflecting beam is then reflected a second time in a switch having a geometry as set forth above, the resulting beam will be restored to a circular or near circular cross-section with enhanced optical density and potentially reduced losses relative to insertion into an output fiber. The recognition of two separate symmetrical geometries for achieving such advantages provides improved options for optically efficient switch configurations.